Oops. Now I need a manual for pushrod 250 head removal
 

Sigh. Like the title says....

Started my Baja X250 and while it was warming up, the throttle stuck open. Took me 4 or 5 seconds trying to get the throttle to release before I killed the motor. Tapped on the carb in the area of the throttle slide and it released. (!??) Started bike again, and carb again stuck open. Wasted 4 or 5 seconds dicking around before shutting it down.

I didn't think it was really revving up that bad, but now it won't start. Did some rudimentary troubleshooting and could hear the motor 'chuffing' back thru the carb and have discovered that motor is only making 30psi compression.

Sigh. Must've bent a valve.

I haven't yet done any disassembly, but first up will be pulling the valve cover and having a look. See if I see huge excess valve clearance on the intake valve.

So, anyone got any pointers on head removal? This is a pushrod 250 motor.

Anyone got a line on a new intake valve and a head gasket?

Sigh.

http://josemaco.files.wordpress.com/...h-62ktjam5.pdf
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As i said b4 in http://chinariders.net/showthread.php?p=181217. Some maker DON'T use mainstream parts. Measure parts is a MUST after disassembled.
Sometimes "chopsticks" is f&%@#d (NOT valve)http://gd3.alicdn.com/imgextra/i3/22...2209511676.jpg

Excellent point, HB.

Thanks for the link and input.

Finally got to working on this. Pulled the engine out Sunday afternoon. Only a handful of bolts and it's right out on the bench. Had to do a bit of cleaning though. Still covered in Georgia clay from the last outing.

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Pulled the valve cover off - nothing unusual at all. Valve clearances all look good. Valves look to open and close properly. Looking in thru the intake and exhaust ports doesn't show anything unusual at all. No obviously bent valve or anything. Hmm.... :hmm:

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So, off comes the head. These pushrod motors are a lot easier than OHC motors in that regard. :) Unfortunately, no smoking gun there, either. No evident 'ding' in the top of the piston, nor anything unusual looking at the valves in the combustion chamber. :hmm:

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So, to check the valve seats, with the valves closed under spring force, I pour acetone in the ports and look for leaks into the combustion chamber. Good sealing valves should not show any seepage for minutes on end.

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For the exhaust, all looked good. Dry for 3 ~ 4 minutes until I dumped the acetone out. For the intake, not so good. Drips appeared immediately, even though to the naked eye all looked good. I only had to watch for 30 seconds. I think I've got my smoking gun as to why I could only develop 30psi compression.

Actually, thinking about it, 30psi is a weird value - one would usually expect good compression or zero compression, not something in between. What I don't know is WHY the valve stopped sealing. A bent valve (my original guess) would give zero, not 30psi. And the valve doesn't look burned although I haven't inspected it out of the head yet.

So, that's where I am now. Next steps will be to see if I can lap the intake valve and get good seating that way, or else I'll have to buy a new intake valve. I probably should just buy the new valve anyway and lap it once it gets here. Haven't even tried to source one yet.

I checked the pushrods and they DO have just the tiniest bit of bow to them. I rolled them across a sheet of mirror glass I have, and you can just barely see a tiny bit of bow. I don't imagine that it's significant though.

So, thoughts or comments? :tup:

clean everything up real good and see if it was just carbon on the seats but my guess is actually fried rings... or at least a stuck ring.... thats what was wrong with my 200cc pushrod motor when I got the bike... dropped a new piston and rings in and fired right up... mine only made 50psi fwiw...

Could be, could be.

The thing is, it ran fine one moment, and not at all the next. Would stuck, broken or worn-out rings act like that? I can see it if I got the bike and it wasn't making compression when I got it. But not with it running as good as it had been for me up to that point.

I'm faced with a minor dilemma, though. I wasn't planning to pull the cylinder off at this point (no base gasket, etc). Looking at the cylinder walls shows virtually no wear. The motor has less than 25 hours on it, total. If I don't pull the cylinder off, and the valve lapping doesn't cure the issue, then I'm out a head gasket and the time to go back into it. Sigh.

I'll see what happens after cleaning and lapping the valves. Maybe I'll put it back together temporarily with the old head gasket just to check the compression after lapping.

I'm gonna ask a stupid question... did you check the compression with the carb off/carb slide all the way open? that can greatly skew the compression numbers.... as for gaskets.... I paper gaskets get replaced with red rtv sealant and metal gaskets get a shot of copper sealant spray and I havent had any issues whatsoever running them...

For mainstream air cooled CG250:
http://item.taobao.com/item.htm?id=14936130630

Excellent - thanks for the link to the parts. Guess I'll have to read up on that sticky about ordering parts from there. :tup: It's amazing - the little currency conversion thingy says the pair of valves costs $4.68. I waste more money than that in vending machines for cryin' out loud. :lol:

Re: the compression testing - when the issue first occurred, that is, when it wouldn't start after the throttle sticking, I could hear 'chuffing' back thru the carburetor while trying to e-start it. At that point, the carb was on, but the airbox connector tube was disconnected and pulled back so I could see into the carb. That sound is what first made me go get the compression tester, because the sound indicated to me a leaky intake valve during compression stroke. I was originally thinking I'd fouled a plug or wasn't getting gas or something simple.

So, first test was with carb on - one time with throttle wide open (and no choke) and repeat check with throttle slide completely removed! :)

And, 'cause I'm anal that way and this IS a cheapie Harbor Freight compression test kit, I went and tested my Yamaha XT225, just to be sure the gauge & tubing was working right and got 135psi or something high up like that.

Once I got the engine out on the bench, I used a big battery charger and ran the e-start and rechecked the compression while watching right in the intake port - no carb or even intake manifold. Same thing at ~30psi.

:clap: http://www.aliexpress.com/item/Chine...030049425.html

I'll be most interested to see what the valve looks like, once you remove it.

How much of a bow did you see in the pushrods? Can you measure it?

Took the intake valve out last night, but didn't get any pictures of it nor the seat. It looked good, though. If you'd handed me the valve alone and said 'have a look at this valve', I'd have said it was fine. Virtually no deposits built up on the valve. Valve seat looked the same - no evidence of damage or nicks, etc.

I did a very brief lapping job and got what looked like good contact all around. The only thing unusual was the lap band was pretty wide - basically the full width of the valve contact edge. I'm used to seeing the contact band a little narrower.

I put it back together and unfortunately, acetone still leaked thru immediately. So, I'll do more lapping and try again. I'll get some pictures this evening.

The head of the valve was perfectly flat, and there didn't appear to be any runout - ie, stem not bent as far as I could tell. This goes along with no evidence of impact on the piston.


As for the pushrods, I don't have V-blocks nor a dial indicator to check properly, but my pushrods have a different look than the picture Humanbeing showed. My pushrods have larger ends and a smaller, 'necked-down' diameter over their length. When I roll the pushrods on the glass, you can just barely see that there's a bow. I suppose I could use feeler gauges to sort of measure the gap, but I'd say for sure less than 1/32". Maybe about 0.010"?


Hopefully this evening, I'll get it lapped properly. If not, I'll have to order a new valve I guess.

hmm if lapping the first time didnt get it I wonder about small hard to see cracks in the seat?

Didn't get to do any lapping last evening, but here are pictures from the 1st session. This is only about 3 ~ 4 minutes lapping, using the suction cup tool thingy and lapping compound. As I say, it still weeped acetone.

I think with a bit more, the valve will seal.

Sure is weird, though, how it went from running like a champ to not running at all. I brought it home from a great dual-sporting trip to Suches, GA area, it sat in the garage for a couple weeks, then I started it up and it was warming up fine 'till I blipped the throttle and it stuck.


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That is a nice, wide margin. Is it possible that one of the valves stuck open? How are the guides? I'm just not seeing any evidence that points to such a massive drop in compression.

Good point - I'll have to inspect up in the valve guides. Nothing obvious on the valve stems, though.

And, since I'm not convinced I've really got a smoking gun even yet, I'll go ahead and pull the cylinder off and inspect the piston / rings. Cylinder walls look good as far as I can see, but I could have a stuck ring(s).

Meanwhile, my new head gasket shipped and says it should be here by Tuesday. So, hopefully by mid-next week, I'll have it back together and see if I have compression.

:tup:

silly quesfion... the "chuffing" you heard.... are you positive it was from the carb, and not maybe from the crankcase vent? just thought of that... that was the first sign I had issues.... I had a reLly high crankcase pressure.... and come to think of it the walls of my cyl looked fine as well...

That's a good thought too. No, not 100% sure, but my breather feeds into the airbox, and I did have the rubber carb-to-airbox hose pulled back off the carb and thought it was coming from the carb.

But you're right, and that's another good incentive for me to pull the cylinder and inspect the piston and rings.

if you have it down that far you may as well do em anyways... I got the piston and a set of rings for around $15-20 off ebay.... cheap insurance at this point....

And, it's bogieboy for the win!!

Lapped the intake valve and got it to hold acetone in the port for a good 5 minutes with no drips.

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So, next step was to pull the cylinder off. Yep, ALL 3 RINGS are stuck fast in their grooves. I've never seen such a thing in a motor with so few hours / hasn't been sitting inert for years. Huh.

Can't even get the oil ring (3-pieces - upper, lower and center wavy piece) to move. Got one bit of the 2nd ring to pop free a bit, but hard to even tell where the end gap IS on the first ring.

So, new piston, ring, wrist pin, circlip set plus base gasket on it's way. Only $20 for the set and should be here next weekend or so.

Cylinder still looks good, but I'll give it a quick hone before the new parts go in.

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and now we know the cause of the loss of compression....LOL my bike only had 320 mi on it when I got it and thats how it came to me... wonder about the piston expanding too quickly from the stuck throttle Nd the rings getti g stuck from that? who knows....

I would imagine acetone would cut the varnish that is holding those rings in.

We can guess the maker from stamped logo & letter
Examples http://item.taobao.com/item.htm?id=42153157591 | http://item.taobao.com/item.htm?id=40158406458 of the mainstream player. Some cheap one that NEVER heard will f***ed in no time

I see some striations on the piston, but nothing to worry about. If you get a decent piston and rings, you should be good to go. Like you, I'd hone the cylinder first (I use a master cylinder hone for that).

I would imagine acetone would cut the varnish that is holding those rings in.


wonder about the piston expanding too quickly from the stuck throttle and the rings getting stuck from that? who knows....



So, now I have some additional food for thought: I was able to remove the 2nd ring and the oil control ring(s) last night, and it appears they were being held in place by grit. When I got them out, it was clear that a very fine grit was all in the ring grooves and was acting as a wedge, holding the rings stuck in place. I cleaned the rings and the grooves a bit, and they seemed to move freely again! The top ring is still resolutely stuck in place, though.

That got me thinking about the original cause of the sticking carb (which I hadn't really gone into at all) - yep, the whole inside of the carb throat, including the throttle slide and it's bore contained the same fine grit. It was clearly the cause of the throttle slide sticking.

Hmmm.... where did that come from? Back to the airbox - yep, the air filter foam was degenerated and had left a large gap at the bottom of it's contact area. And of course, on this bike, there's no airbox cover. The top of the airbox is wide open, and when the seat is installed, it doesn't really prevent the back tire from flinging mud/dirt up over the top of the battery and into the airbox. The inside bottom of the battery box and the airbox was filled with fine sand in all the corners.

So, I think the poor motor was sucking in sand and it jammed both the throttle slide and the piston rings. I also think it might have screwed up the intake valve sealing. And, I think that's what gives the piston sides that 'sandblasted' look, as opposed to the usual wear marks one sees.

So, always be sure to check your air filter! I think when this goes back together, I might remove the airbox-to-carb rubber tube and just install a 2-stage foam pod type filter.

And, I'm going to try to figure out a shield or deflector for under the seat to help keep the tire from just flinging mud right into the area.

Thoughts and comments?

Excellent detective work. Now you know why I don't use the stock airbox on any of my China stuff. Here's what has worked for me: http://www.ebay.com/itm/Red-42mm-Ang...e5ec4c&vxp=mtr

Success!

Got the bike put back together Sunday. New piston, rings and gaskets showed up Friday. Honed the cylinder and put it all back together. Got the motor back in the bike and enough stuff back together to hear it run for 5 ~ 10 seconds on a shot of spray start. Woo Hoo!

At one point, I was turning it over with the e-start to get oil pumped up to the head, and I did a quick compression test, even though it's never been fired and was cold. It made about 150psi, so I'm well pleased. Shop manual says 178psi, but that's warm and with seated rings.

As for air filter, I used a Uni-style foam universal. All I did was remove the stock carb-to-airbox rubber tube. Fits well, so I'm hopeful this won't happen again. Especially when I fabricate a few bits to help deflect rear tire mud flinging.

So, there we go. Lesson learned. Thanks to everyone for links, support and discussion.

Kirk

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That's awesome news! You might need a larger main jet with that new filter.